Radio-selected BL Lac Objects Research Articles

Characteristics of the Radiation from Blazars

We have collected short-timescale variability data of 47 blazars, estimated the masses of their central black holes and the sizes of their radiation regions at different wavebands, and made a statistical analysis on the calculated results. It is found that the central black hole mass of blazars falls in the range 10 7M ⊙ to 10 10M ⊙, and that the BL Lac objects and the flat-spectrum radio quasars have very different central black hole masses (the latter being generally greater), while they have very similar sizes of radiation regions in the infrared and γ-ray wavebands. Also, using the collected bolometric luminosity data, we have analyzed the relationship between the bolometric luminosity of blazars and their short-timescale variability, and it is concluded that the radiations from the radio-selected BL Lac objects (RBLs) and flat-spectrum radio quasars (FSRQs) are strongly beam-confined, while the effect of relativistic beaming is relatively small for the X-ray-selected BL Lac objects (XBLs).

A Statistical Study of XBLs, RBLs and FSRQs at 1.5 GHz

BL Lac objects are similar to theflat spectrumradioquasarsin manyaspects except regarding the emission lines. In order to study their relationship, we selected 56 BL Lacertae objects (33 X-ray-selected, 23 radio-selected) and 45 flat spectrum radio quasars, analyzed their radio luminosities and core-dominanceparameters. We found that the radio luminosities of the radio selected BL Lac objects located in between the X-ray selected BL Lac objects and the flat spectrum radio quasars. However, this intermediate position does not hold for the core-dominance parameter: the RBLs have the largest core-dominanceparameters. This sug- gests that the core-dominance parameter can not be taken as a sequencing criterion. We also investigatedthe correlationbetween the luminosity and the core-dominanceparameterfor the three subclasses. We concluded that, here, the sequence XBL-RBL—FSRQ still exists.

Highly Polarized Optically Selected BL Lacertae Objects

Observations of candidate BL Lacertae objects spectroscopically selected from the Sloan Digital Sky Survey (SDSS) reveal a large fraction with high polarization (P > 3%). This result confirms that synchrotron radiation makes an important contribution to the observed optical continuum for most objects in the sample. The SDSS sample can be divided into separate categories, with objects of undetermined redshift generally having the highest optical polarization. Polarization as high as 23% and the lack of spectral features suggests that the synchrotron continuum completely dominates the spectra of these sources. The mean polarization levels observed for objects having measured redshifts is much lower, with the maximum observed polarization for this group being ~10%. The lower polarizations of these objects are reminiscent of the less spectacular polarization levels shown by BL Lac objects discovered in X-ray surveys. We find no SDSS BL Lac candidates at z > 1 with P > 3%, calling their classification as BL Lac objects into question. In addition, the existence of radio-quiet BL Lac objects is not verified since none of 10 potentially radio-weak BL Lac candidates observed are highly polarized. Regardless of whether the high-redshift and radio-weak objects are included in this optical sample, the overall levels of polarization observed are intermediate between those seen for X-ray and radio-selected BL Lac objects.

On the difference between the X-ray selected and the radio selected BL Lac objects in the near infrared

By using results from the 2MASS observation, the differences between the X-ray selected BL Lac objects (XBLs) and the radio selected ones (RBLs) in the near infrared are discussed in this paper. It is found that, statistically, the RBLs have redder near infrared colors and steeper spectral indices in the near infrared than XBLs, whereas the XBLs have more influences from their host galaxies than the RBLs. It is also seen that the RBLs have much brighter luminosity in the near infrared than the XBLs. In addition, the high polarization characteristics are found in common for RBLs, but not for XBLs.

The HRX-BL Lac sample – Evolution of BL Lac objects

The unification of X-ray and radio selected BL Lacs has been an outstanding problem in the blazar research in the past years. Recent investigations have shown that the gap between the two classes can be filled with intermediate objects and that apparently all differences can be explained by mutual shifts of the peak frequencies of the synchrotron and inverse Compton component of the emission. We study the consequences of this scheme using a new sample of X-ray selected BL Lac objects comprising 104 objects with z<0.9 and a mean redshift z=0.34. 77 BL Lacs, of which the redshift could be determined for 64 (83%) objects, form a complete sample. The new data could not confirm our earlier result, drawn from a subsample, that the negative evolution vanishes below a synchrotron peak frequency log (peak-frequency) = 16.5. The complete sample shows negative evolution at the 2 sigma level (<V_e/V_a> = 0.42 +- 0.04). We conclude that the observed properties of the HRX BL Lac sample show typical behaviour for X-ray selected BL Lacs. They support an evolutionary model, in which flat-spectrum radio quasars (FSRQ) with high energetic jets evolve towards low frequency peaked (mostly radio-selected) BL Lac objects and later on to high frequency peaked (mostly X-ray selected) BL Lacs.

High-Resolution Radio Imaging of Gravitational Lensing Candidates in the 1 Jansky BL Lacertae Sample

While BL Lacertae objects are widely believed to be highly beamed, low-luminosity radio galaxies, many radio-selected BL Lac objects have extended radio power levels and optical emission lines that are too luminous to be low-luminosity radio galaxies. In addition, Stocke & Rector discovered an excess of Mg II absorption systems along BL Lac sight lines compared with quasars, suggesting that gravitational lensing may be another means of creating the BL Lac phenomenon in some cases. We present a search for gravitationally lensed BL Lac objects with deep, high-resolution, two-frequency VLA radio maps of seven lensing candidates from the 1 Jy BL Lac sample. We find that none of these objects are resolved into an Einstein ring like B0218+357, nor do any show multiple images of the core. All of the lensing candidates that were resolved show a flat-spectrum core and very unusual, steep-spectrum extended morphology that is incompatible with a multiply lensed system. Thus, while these observations do not rule out microlensing, no macrolensing is observed.

Intermediate BL Lac objects

The 200-mJy sample, defined by Marcha et al., contains about 60 nearby, northern, flat-spectrum radio sources. In particular, the sample has proved effective at finding nearby radio-selected BL Lac objects with radio luminosities comparable to those of X-ray-selected objects, and low-luminosity flat-spectrum weak emission-line radio galaxies (WLRGs). The 200-mJy sample contains 23 BL Lac objects (including 6 BL Lac candidates) and 19 WLRGs. We will refer to these subsamples as the 200-mJy BL Lac sample and the 200-mJy WLRG sample, respectively. We have started a systematic analysis of the morphological pc-scale properties of the 200-mJy radio sources using VLBI observations. This paper presents VLBI observations at 5 and 1.6 GHz of 14 BL Lac objects and WLRGs selected from the 200-mJy sample. The pc-scale morphology of these objects is briefly discussed. We derive the radio beaming parameters of the 200-mJy BL Lac objects and WLRGs and compare them with those of other BL Lac samples and with a sample of FR I radio galaxies. The overall broad-band radio, optical and X-ray properties of the 200-mJy BL Lac sample are discussed and compared with those of other BL Lac samples, radio- and X-ray-selected. We find that the 200-mJy BL Lac objects fill the gap between HBL and LBL objects in the colour–colour plot, and have intermediate αXOX as expected in the spectral energy distribution unification scenario. Finally, we briefly discuss the role of the WLRGs.

The Doppler Effect and Spectral Energy Distribution of Blazars

The relativistic beaming model is adopted to discuss quantitatively the observational differences between radio-selected BL Lac objects (RBLs) and X-ray-selected BL Lac objects (XBLs), and between BL Lac objects and flat spectrum radio quasars (FSRQs). The main results are the following: (1) In the Doppler corrected color-color (alpha (in)(ru) - alpha (in)(ox)) diagram, XBLs and FSRQs occupy separated regions, while RBLs bridge the gap between them. These properties suggest that similar intrinsic physical processes operate in all the objects under a range of intrinsic physical conditions. (2) Our results are consistent with the results of Sambruna, Maraschi and Urry (1996) from other methods. We show the alpha (xox) introduced by Sambruna to be a good index for describing the energy distribution because it represents the intrinsic energy distribution and includes the Doppler correction. (3) The Doppler effect of relativistic beaming is the main mechanism, and the physical differences (such as magnetic fields, electron energies) are also important complementary factors for understanding the relation between XBLs and RBLs.

Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample

Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 high-energy cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 ‘intermediate’ sources. Data for LBLs, which are mostly radio-selected BL Lac objects (RBLs), are taken from the literature. It is shown that the radio—millimetre spectral indices of HBLs are slightly steeper than those of the LBLs . A correlation exists between α5–230 and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties, it is also consistent with the predictions of existing unified schemes that relate BL Lac objects to Fanaroff—Riley class I radio galaxies. The HBLs have significantly flatter submillimetre—X-ray spectral indices than the LBLs although the two intermediate sources also have intermediate values of α230-X∼-0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α230-X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs.

Photometric monitoring of three BL Lacertae objects in 1993-1998

The results of optical photometric (BV RI) monitoring of three BL Lac objects over a time interval of about four years are presented. The sources are three classical radio-selected BL Lac objects, BL Lac, OJ 287 and PKS 0735+178. During our observation OJ 287 was in the stage of a large periodic outburst which consisted of at least two peaks. Almost all the observations obtained over consecutive nights detected intranight variations. In 1995 and 1996 BL Lac kept in faint states, with fewer and smaller rapid flares and fluctuations. On the contrary, in late 1997 BL Lac was at the stage of a large outburst, accompanied with much more large amplitude rapid flares and fluctuations. PKS 0735+178 was almost at its faint end from 1994 to early 1998. Over this time interval, the intraday variations and microvariations in PKS 0735+178 were rare and the amplitude was very small, except a rapid darkening of similar to 0.4 mag on 24 January 1995. Previous work by Webb et al. (1988); Wagner et al. (1996); Plan et al. (1997) also showed the same behaviour of variability as BL Lac and PKS 0735+178 in BL Lac, S5 0716+714, PKS 2155 - 304, respectively. We propose that the motion of orientation of the relativistic jet in a BL Lac object be responsible for these variability behaviours.

The intraday variability in the radio-selected and X-ray-selected BL Lacertae objects

Seven BL Lac objects have been photometrically observed in an effort to study the difference of optical intraday variability between the radio-selected BL Lac objects (RBLs) and X-ray-selected BL Lac objects (XBLs). The objects we observed are selected arbitrarily. They are four RBLs; PKS 0735+178. PKS 0754+101, OJ 287 and BL Lac, and three XBLs, H 0323+022, H 0548-322 and H 2154-304. During the observation all of them exhibited microvariation. and H 0323+022 and H 0548-322 sometimes showed brightness oscillation. PKS 0735+178 and BL Lac were in their faint states and not very active. It seems that RBLs do not show microvariability more frequently than XBLs.

Decijansky Radio Flux BL Lacertae Objects

BL Lac objects with radio flux in the range of [1, 10) decijanskys (dJy) are defined as observational class dJy. Because the radio fluxes of most radio-selected BL Lac objects (RBLs) are bigger than 1 Jy while the radio fluxes of most X-ray-selected BL Lac objects (XBLs) are less than 1 dJy, we investigate whether the dJy class forms a bridge connecting RBLs and XBLs on the αro-αox color-color diagram. We compose a dJy sample by collecting dJy class sources from radio and X-ray surveys as well as other means of observation. Our study shows that the dJy sample so collected does not form a bridge connecting RBLs and XBLs. Instead, the sources of the sample are mainly distributed in the main domains of RBLs and XBLs. We find no evidence of the existence of any specific observational class in the medium of the two classes. This suggests that RBLs and XBLs may not be unified merely through observational orientation, because such a unified scheme expects some observational class to form the bridge. In order to have a more reliable study, it is proposed that 1 dJy radio surveys that include some previously missed BL Lac objects should be made.

Unifying models for X-ray-selected and radio-selected BL Lac objects

We discuss alternative interpretations of the differences in the Spectral Energy Distributions (SEDs) of BL Lacs found in complete Radio or X-ray surveys. A large body of observations in different bands suggests that the SEDs of BL Lac objects appearing in X-ray surveys differ from those appearing in radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at much higher frequency. In order to explain the different properties of radio and X-ray selected BL Lacs Giommi and Padovani proposed a model based on a common radio luminosity function. At each radio luminosity, objects with high frequency spectral cut-offs are assumed to be a minority. Nevertheless they dominate the X-ray selected population due to the larger X-ray-to-radio-flux ratio. An alternative model explored here (reminiscent of the orientation models previously proposed) is that the X-ray luminosity function is "primary" and that at each X-ray luminosity a minority of objects has larger radio-to-X-ray flux ratio. The predictions of the two scenarios, computed via a Montecarlo technique, are compared with the observed properties of BL Lacs in the two samples extracted respectively from the 1 Jy radio survey and the Einstein Slew Survey. We show that both models can explain a number but not all the observed features. We then propose a completely new approach, based on the idea that the physical parameter which governs the shape of the SEDs, is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity we show that the observational properties of the two surveys can be reproduced at least with the same accuracy as the two previous models.

The Unification of Radio-Loud AGN

In a recent review paper we summarized the current status of unification of radio-loud AGN (Urry &amp; Padovani 1995 PASP 107, 803), connecting high-luminosity (FR II) radio galaxies with quasars, and low-luminosity (FR I) radio galaxies with BL Lac objects. Unified schemes are motivated by the knowledge that AGN appearance depends strongly on orientation (Fig. 1): optical/UV light from the centers of many AGN is obscured by circumnuclear matter, and in radio-loud AGN, bipolar relativistic jets beam light along the jet axes. Understanding these radiation anisotropics allows us to unify apparently distinct classes of AGN that differ primarily because of orientation.Our review described the classification and general properties of AGN and summarized the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming. We outlined the evidence, both observed isotropic properties and statistical arguments, for connecting FR IIs with quasars and FR Is with BL Lacs. The population statistics (with beaming) are in accordance with available data and suggest γ ≃ 5 for low-luminosity AGN and γ ≃ 10 for high-luminosity AGN. The distinctions between X-ray-selected and radio-selected BL Lac objects, and between BL Lacs and flat-spectrum variable quasars, still not understood, provide clues to the underlying physics of blazars. Our review discussed several possible problems and complications, and concluded with a list of the ten questions we believe are the most pressing in this field.

Soft X-Ray Observations of a Complete Sample of X-Ray--selected BL Lacertae Objects

view Abstract Citations (60) References (84) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Soft X-Ray Observations of a Complete Sample of X-Ray--selected BL Lacertae Objects Perlman, Eric S. ; Stocke, John T. ; Wang, Q. Daniel ; Morris, Simon L. Abstract We present the results of ROSAT PSPC observations of the X-ray selected BL Lacertae objects (XBLs) in the complete Einstein Extended Medium Sensitivity Survey (EM MS) sample. None of the objects is resolved in their respective PSPC images, but all are easily detected. All BL Lac objects in this sample are well-fitted by single power laws. Their X-ray spectra exhibit a variety of spectral slopes, with best-fit energy power-law spectral indices between α = 0.5-2.3. The PSPC spectra of this sample are slightly steeper than those typical of flat ratio-spectrum quasars. Because almost all of the individual PSPC spectral indices are equal to or slightly steeper than the overall optical to X-ray spectral indices for these same objects, we infer that BL Lac soft X-ray continua are dominated by steep-spectrum synchrotron radiation from a broad X-ray jet, rather than flat-spectrum inverse Compton radiation linked to the narrower radio/millimeter jet. The softness of the X-ray spectra of these XBLs revives the possibility proposed by Guilbert, Fabian, & McCray (1983) that BL Lac objects are lineless because the circumnuclear gas cannot be heated sufficiently to permit two stable gas phases, the cooler of which would comprise the broad emission-line clouds. Because unified schemes predict that hard self-Compton radiation is beamed only into a small solid angle in BL Lac objects, the steep-spectrum synchrotron tail controls the temperature of the circumnuclear gas at r ≤ 1018 cm and prevents broad-line cloud formation. We use these new ROSAT data to recalculate the X-ray luminosity function and cosmological evolution of the complete EMSS sample by determining accurate K-corrections for the sample and estimating the effects of variability and the possibility of incompleteness in the sample. Our analysis confirms that XBLs are evolving "negatively," opposite in sense to quasars, with Ve/Va = 0.331±0.060. The statistically significant difference between the <Ve/Va> values for X-ray and radio-selected BL Lac objects remains a difficulty for models which unify these two types of objects. We have identified one addition to the sample, so that the sample now has 23 objects. We find no evidence for a substantial number of unidentified low-luminosity BL Lac objects hidden in our sample, as had been suggested by Browne & Marcha (1993) although a few such objects may be present. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176671 Bibcode: 1996ApJ...456..451P Keywords: GALAXIES: BL LACERTAE OBJECTS: GENERAL; GALAXIES: EVOLUTION; GALAXIES: LUMINOSITY FUNCTION; MASS FUNCTION; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (42) NED (27) HEASARC (1) Related Materials (1) Erratum: 1996ApJ...465.1010P

The connection between x-ray- and radio-selected BL Lacertae objects

view Abstract Citations (617) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Connection between X-Ray-- and Radio-selected BL Lacertae Objects Padovani, Paolo ; Giommi, Paolo Abstract We explain the properties of X-ray-selected BL Lacertae objects, under the assumption that they constitute the small minority of the BL Lac population with energy cutoff located in the UV/X-ray band as suggested by their multifrequency spectra. In particular, we derive their X-ray luminosity function, log N-log S, and radio flux distribution starting from radio-selected BL Lac objects using a method that does not require any free parameters. Our hypothesis is at variance with the most common explanation for the existence of two classes of BL Lac objects, namely, that X-ray-selected objects are viewed significantly off the beaming axis while radio-selected ones have their jets more aligned with the line of sight. In this case, in fact, X-ray-selected sources should be intrinsically more numerous than objects selected in the radio band at a fixed value of some isotropic luminosity such as that of the diffuse radio emission. While also accounting for most of the properties of BL Lac objects detected in current X-ray surveys, this latter scenario would predict their radio emission to be dominated by the extended (unbeamed) component, in contrast with observations. We also address the question of the negative evolution apparently exhibited by X-ray-selected BL Lac objects, showing that neither recognition problems nor a break in the X- ray spectrum are likely to be responsible for it. Our hypothesis explains in a simple way the bimodal distribution of BL Lac objects in the α_ro_0α_ox_ diagram. It also makes specific predictions for the X-ray, radio, and optical properties of BL Lac objects which will be tested by future deep X-ray surveys. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175631 arXiv: arXiv:astro-ph/9412073 Bibcode: 1995ApJ...444..567P Keywords: Bl Lacertae Objects; Optical Properties; Radio Astronomy; Spectrum Analysis; X Ray Astronomy; Emission Spectra; Energy Spectra; Heao 2; Luminosity; Red Shift; Spatial Distribution; Astronomy; GALAXIES: BL LACERTAE OBJECTS: GENERAL; RADIO CONTINUUM: GALAXIES; X-RAYS: GALAXIES; Astrophysics E-Print: 49 pages, PostScript paper and figure files. To appear in The Astrophysical Journal. Send all Comments, questions, requests, to padovani@roma2.infn.it full text sources arXiv | ADS | data products NED (6) SIMBAD (3)

The X-ray spectra of blazars observed with EXOSAT

A systematic analysis of the X-ray spectra (0.1-10 keV) of all blazars observed with EXOSAT is presented. The objects include 16 X-ray-selected BL Lac objects (XBLs), five radio-selected BL Lac objects (RBLs), and five highly polarized quasars (HPQs). Comparing the results of single-power-law fits for the different classes, we find that XBLs have steeper X-ray spectra than HPQs (average photon indices &lt;{GAMMA}&gt; ~ 2.20_-0.15_^+0.17^ and 1.62_-0.24_^+0.11^, respectively). The X-ray luminosities of XBLs and RBLs are similar, while HPQs tend to be somewhat more luminous. For the three groups, a correlation between the average medium-energy luminosity and the average spectral index is found, in the sense that sources of higher luminosity have flatter spectra. For repeatedly observed objects (XBLs), the spectra harden systematically with increasing intensity. For seven (out of 16) XBLs, corresponding to the brightest objects in the sample, the X-ray spectrum is significantly better described by a convex, broken-power-law model than by a single power law. The results are discussed in the framework of current models for blazars.

A quantitative model of coherent, stimulated emission in astrophysical jets

On the basis of issues raised by observations of BL Lac objects and the qualitative jet model proposed by Baker et al. in 1988, we have been led to consider the quantitative role of coherent, stimulated emission in jets and construct a new jet model of blazars in which a relativistic electron beam with an axial symmetric, power-law distribution is injected from the central engine into the jet plasma. We study quantitatively the synchrotron emission of the relativistic electron beams. Using the weak turbulent theory of plasma, we discuss the interaction between relativistic electron beams and jet plasma, and the roles of stimulated emission. The main results are: (1) The synchrotron emission increases sensitively with the increase of the angle between the direction of the beam and the magnetic field. When the direction of the beam is vertical to the magnetic field, the synchrotron emission reaches its maximum, i.e. the emitted waves are beamed in the direction of the jet axis. We suggest that radio selected BL Lac objects belong to this extreme classification. (2) The synchrotron emission of the relativistic beam increases rapidly with the increase of the Lorentz factor of the relativistic electron, gamma, when gamma less-than-or-equal-to 22.5, then decreases rapidly with increase of (3) The stimulated emission also increases with increasing Lorentz factor gamma of the relativistic electrons when gamma less-than-or-equal-to 35 and then decreases with the increasing gamma. The maximum stimulated emission and the maximum synchrotron emission occur at different frequencies. Stimulated emission is probably very important and reasonable flare mechanism in blazars. (4) The rapid polarization position angle (PA) swings may arise from the interaction between the relativistic electron beam and the turbulent plasma.

BL Lac objects and acceleration model

In this paper, the acceleration model is applied to data from RBLs (Radio Selected BL Lac objects). The corrected data from RBLs are found to be nearly the same as those from XBLs (X-Ray Selected BL Lac objects), the expected result, that points of U - B, B - V of XBLs and RBLs occupy the same region in the (B - V)-(U - B) diagram has been obtained, and the Radio Doppler factors obtained by us are highly consistent with those found by others.

First Ultraviolet Spectropolarimetry of Radio-selected BL Lacertae Objects

We present ultraviolet spectropolarimetry of the BL Lac objects OJ 287 and 0754+100 (OI 090.4) acquired with the Faint Object Spectrograph of the Hubble Space Telescope. These are the first such observations of radio-selected BL Lac objects and the polarimetry spans the wavelength range 1580-3300 A in the observer's frame. Neither object exhibited emission-line features in their UV spectra. OJ 287 was very faint during the UV observations (V ~ 16). The UV linear polarization of this object was ~20%. There is no indication that the degree of polarization or the polarization position angle varies with wavelength. Nearly simultaneous optical ground-based measurements show that the polarization does not significantly change with wavelength out to ~8000 A. The UV and optical polarization of 0754+100 was ~8% and also wavelength-independent during the epoch of observation. These observations confirm that the synchrotron emission that dominates the optical continuum also dominates in the ultraviolet in these two BL Lac objects. There is no evidence for significant contributions to the UV/optical flux by other emission components, and we set limits on the brightness of nonsynchrotron continuum components.